In a semiconductor having a thin-film-laminated structure produced by a so-called “metal organic chemical vapor deposition method” (hereinafter, referred to simply as an “MOCVD method”), the improvement of sharpness in the interface between thin films of the laminated structure, formed by epitaxial growth, is an indispensable requirement to improve the quality of the semiconductor. In other words, in order to improve sharpness in an interface between thin films, the total amount of gas flow supplied to a reactor is required so as not to be changed even if the kind of source gases is changed, such as by a switching operation from one source gas to another source gas for the vapor-phase growth reaction, and it is necessary to sharply perform the switching operation between the source gases. In other words, in order to improve sharpness in the interface between thin films, it is necessary to maintain a constant total amount of gas flow supply to the reactor even when it is necessary to switch the source of gas flow supplied from one source gas to another source gas.
To do so, first, the amount of gas flow must be controlled with high accuracy and with high responsibility so as not to cause a change in gas pressure within the inside of the reactor, or within the inside of pipes, when switching is performed between source gases. To meet these requirements, many gas supply systems, such as configured in FIG. 10, have been conventionally used in semiconductor manufacturing equipment that adopt the MOCVD method.
In more detail, FIG. 10 shows an example of a basic configuration of a conventional gas supply system that adopts the MOCVD method. In FIG. 10, L1 is a main gas supply line, L2 is a vent gas line, PC is a reactor (process chamber), VP is a vacuum pump, MFC1, MFC2, and MFC are mass-flow controllers, respectively, P1 to P4 are pressure detectors, respectively, P1s is a pressure detection signal, DP is a differential pressure detector, ΔP is a differential pressure detection signal, VR1, VR2, VR3, and VR4 are pressure regulators, respectively, V0, V1, V2, V3, V4, and V are control valves, respectively, A1 and A2 are gas supply mechanisms, respectively, B1 and B2 are change-over valve mechanisms, respectively, OM is an organometallic liquid, GB is a gas cylinder, C0 to C5 are carrier gases, respectively, and CA and CB are source gases, respectively.
Let it be supposed, hypothetically, that the set value of the amount of flow of the mass-flow controller MFC1 of the main gas supply line L1 and that of the mass-flow controller MFC2 of the vent gas supply line L2 are set at Q1 and Q2, respectively, and the pressure value of the reactor PC are set, and carrier gases C01 and C02 are allowed to flow to the lines L1 and L2, respectively, at this moment. Furthermore, source gas supply mechanisms A1 and A2 are set and operated, and the amount of flow of the source gas CA and that of the carrier gas C3 are adjusted to have the same value, and the amount of flow of the source gas CB and that of the carrier gas C5 are adjusted to have the same value. Furthermore, valve switching mechanisms B1 and B2 are operated, valves V1a and V2b are opened, valves V1b and V2a are closed, valves V1a and V4b are opened, and valves V3b and V4a are closed in this hypothetical state.
As a result, gas consisting of source gas CA+source gas CB+carrier gas C01 (i.e., the amount of flow of these gases is equal to the set value Q1 of the mass-flow controller MFC1) flows through the main gas supply line L1, and is supplied to the reactor PC. On the other hand, the carrier gas consisting of carrier gas C5+carrier gas C3+carrier gas C02 (i.e., the total amount of flow of these gases is equal to the set value Q2 of the mass-flow controller MFC2) flows through the vent gas supply line L2, and is evacuated through a vacuum pump VP.
If the supply of the source gas CA is stopped from this hypothetical state described above, and, instead of this gas CA, a source gas CD (not shown) is supplied to the main gas supply line L1 (i.e., the source gas CA is changed to a source gas CD (not shown)) via a change-over valve mechanism B3 (not shown) from another source gas supply mechanisms A3 (not shown), the valve V1a is first closed, the valve V1b is opened, the valve V2b is closed, and the valve V2a is opened.
If a difference in pressure occurs between the gas flowing through the main gas line L1 and the gas flowing through the vent gas supply line L2 when these valves V1a, V1b, V2b, and V2a are switched, a transient response will be caused subsequently in the amount of flow of the source gas CD newly supplied to the main gas line L1, and, as a result, sharpness in the interface between an already-formed thin film and a new thin film, formed by the newly-supplied source gas CD, will deteriorate.
Therefore, to reduce the differential pressure ΔP between the lines L1 and L2 to zero, the amount of gas flow running through the vent gas supply line L2 is first set at a predetermined value by means of the mass-flow controller MFC2 and, thereafter, the differential pressure ΔP between the lines L1 and L2 is detected by the differential pressure detector DP and, thereafter, the resulting detection value ΔP is fed back to a pressure regulator VR2 so that the resistance of the vent gas supply line L2 is adjusted, and the above-mentioned differential pressure ΔP becomes zero.
According to another or second method, the differential pressure ΔP detected by the differential pressure detector DP is fed back to the mass-flow controller MFC2 while keeping the set value of the pressure regulator VR2 constant as shown by the alternate long and short dash line of FIG. 10 (i.e., phantom lines), and the amount of gas flow running through the vent gas supply line L2 is thus controlled, and the above-mentioned differential pressure ΔP can be adjusted to zero. Of course, in the case of this second method, to keep the gas pressure of the inside of the main gas supply line L1 constant, the pressure of the inside of the main gas supply line L1 is detected by a pressure detector P1, as shown by the alternate long and short dash line of FIG. 10 (i.e., phantom lines), and the resulting detection value P1s is fed back to the pressure regulator VR1, so that the resistance of the main gas supply line L1 is adjusted, and, thus, the gas pressure of the main gas supply line L1 is kept constant.
In the gas supply system shown in FIG. 10, the differential pressure ΔP between the lines L1 and L2 is adjusted to substantially zero when switching is performed between source gases, and, as a result, so-called “sharpness” in an interface forming between an already-formed thin film and a new thin film formed by a source gas, selected by the switching operation, is excellently maintained, and an excellent effect by which a high-quality semiconductor can be obtained is brought about.
However, many problems to be overcome still remain in the gas supply system of FIG. 10. Among these problems, particularly-problematical points are the following two (I) and (II). First point (I): The differential pressure detector DP, a branch pipe used to mount the differential pressure detector DP, the pressure regulators VR1 and VR2, a feedback control line FBL, etc., are needed, in addition to the mass-flow controller MFC1 and the mass-flow controller MFC2, in order to operate the system. Therefore, the equipment and systems are complicated, and a large space is needed to mount the differential pressure detector DP, thus making it difficult to make the gas supply system compact. Second point (II): There is a need to adjust the internal pressure of the vent gas supply line L2 by means of the pressure regulator VR2 based on the differential pressure detection value ΔP so as to reduce the differential pressure ΔP to zero (alternatively, there is a need to adjust the internal pressure of the main gas supply line L1 by means of the pressure regulator VR1 based on a differential pressure detection value ΔP so as to reduce the differential pressure ΔP to zero), and therefore the responsiveness of pressure control is low.    Patent Literature 1: Japanese Published Patent Application No. 2005-223211.    Patent Literature 2: Japanese Published Patent Application No. H8-288226.